Elsevier

The Lancet Infectious Diseases

Volume 12, Issue 9, September 2012, Pages 687-695
The Lancet Infectious Diseases

Articles
Estimated global mortality associated with the first 12 months of 2009 pandemic influenza A H1N1 virus circulation: a modelling study

https://doi.org/10.1016/S1473-3099(12)70121-4 Get rights and content

Summary

Background

18 500 laboratory-confirmed deaths caused by the 2009 pandemic influenza A H1N1 were reported worldwide for the period April, 2009, to August, 2010. This number is likely to be only a fraction of the true number of the deaths associated with 2009 pandemic influenza A H1N1. We aimed to estimate the global number of deaths during the first 12 months of virus circulation in each country.

Methods

We calculated crude respiratory mortality rates associated with the 2009 pandemic influenza A H1N1 strain by age (0–17 years, 18–64 years, and >64 years) using the cumulative (12 months) virus-associated symptomatic attack rates from 12 countries and symptomatic case fatality ratios (sCFR) from five high-income countries. To adjust crude mortality rates for differences between countries in risk of death from influenza, we developed a respiratory mortality multiplier equal to the ratio of the median lower respiratory tract infection mortality rate in each WHO region mortality stratum to the median in countries with very low mortality. We calculated cardiovascular disease mortality rates associated with 2009 pandemic influenza A H1N1 infection with the ratio of excess deaths from cardiovascular and respiratory diseases during the pandemic in five countries and multiplied these values by the crude respiratory disease mortality rate associated with the virus. Respiratory and cardiovascular mortality rates associated with 2009 pandemic influenza A H1N1 were multiplied by age to calculate the number of associated deaths.

Findings

We estimate that globally there were 201 200 respiratory deaths (range 105 700–395 600) with an additional 83 300 cardiovascular deaths (46 000–179 900) associated with 2009 pandemic influenza A H1N1. 80% of the respiratory and cardiovascular deaths were in people younger than 65 years and 51% occurred in southeast Asia and Africa.

Interpretation

Our estimate of respiratory and cardiovascular mortality associated with the 2009 pandemic influenza A H1N1 was 15 times higher than reported laboratory-confirmed deaths. Although no estimates of sCFRs were available from Africa and southeast Asia, a disproportionate number of estimated pandemic deaths might have occurred in these regions. Therefore, efforts to prevent influenza need to effectively target these regions in future pandemics.

Funding

None.

Introduction

Influenza pandemics are typically characterised by higher, but widely varying, number of deaths than are seasonal epidemics.1 The emergence of pandemic influenza A H1N1 in April, 2009, led WHO to request that countries report all laboratory-confirmed deaths associated with it. For the period up to August, 2010, 18 500 deaths associated with laboratory-confirmed 2009 pandemic influenza A H1N1 have been reported.2 This number is likely to be an underestimate because diagnostic specimens are not always obtained from people who die with influenza and the viruses might no longer be detectable by the time of death in some people.

Estimation of the 2009-pandemic-associated mortality presents several challenges. First, data for influenza in many countries are sparse and obtained through virological surveillance without standardised case reporting or population denominators needed to estimate incidence. Second, the level and timing of the circulation of the pandemic virus might vary by region and country.3, 4, 5 Third, the severity of influenza might vary by region and country due to differences in access to and quality of health care, nutritional status, prevalence of underlying chronic disorders, age distribution of the populations, and the use of influenza vaccines and antiviral drugs.6, 7, 8, 9, 10

Influenza-associated mortality is often estimated indirectly, by use of statistical models, as the number of excess deaths during periods of circulation of the virus.11, 12, 13 Influenza-associated deaths that might be missed by direct counts of only laboratory-confirmed deaths are taken into account with the use of these indirect approaches. However, indirect estimation might not be as easy to apply in settings where influenza viruses circulate perennially without a clearly defined season or where data for circulation with other respiratory viruses are not available. Additionally, methods to estimate excess mortality require the availability of vital statistics data that are sparse in some regions of the world and not always available in aggregate until 2–3 years after deaths are reported in countries that do have data. Thus, new methods are needed to produce timely and representative estimates of worldwide influenza-associated mortality.

A global estimate of the mortality associated with the 2009 pandemic influenza A H1N1 is needed to document the effect of the pandemic on the world's population and to help guide allocation and delivery of prevention and treatment measures during future pandemics. So far, most reported estimates of mortality associated with 2009 pandemic influenza A H1N1 are from high-income, temperate countries.14, 15, 16, 17 We developed a new approach to estimate global mortality and the number of years of life lost (YLL) associated with the first year of circulation of the 2009 pandemic influenza A H1N1 virus in each country.

Section snippets

Estimation of respiratory mortality rate

To calculate the base respiratory mortality rate associated with the 2009 pandemic influenza A H1N1 (unadjusted for differences between regions in risk of death) for each country, we estimated the 12 month cumulative symptomatic attack rate (sAR) with data from high-income, middle-income, and low-income countries and multiplied it by the estimated symptomatic case fatality ratio (sCFR) from select high-income countries. sAR was defined as the percentage of the population who developed a

Results

The total of the median estimates of country-specific respiratory deaths associated with the 2009 pandemic influenza A H1N1 was 201 200 (range calculated by summing the 25th and 75th percentile estimates in each age group in each country), more than ten times the number of global deaths reported to WHO for the period April, 2009, to August, 2010 (table 3). Summation of the fifth and 95th percentile estimates in each age group and country would have resulted in an estimated range of

Discussion

During the first year of circulation of the 2009 pandemic influenza A H1N1 virus in each country, an estimated 105 700–395 600 people died of associated respiratory illness. Addition of cardiovascular deaths associated with 2009 pandemic influenza A H1N1 among people older than 17 years increased the mortality burden to 151 700–575 400 deaths (table 3). Our global estimate was more than 15 times higher than the number of laboratory-confirmed deaths reported to WHO during the first 16 months of

References (54)

  • C Cohen et al.

    Elevated influenza-related excess mortality in South African elderly individuals, 1998–2005

    Clin Infect Dis

    (2010)
  • NP Johnson et al.

    Updating the accounts: global mortality of the 1918–1920 “Spanish” influenza pandemic

    Bull Hist Med

    (2002)
  • G La Ruche et al.

    The 2009 pandemic H1N1 influenza and indigenous populations of the Americas and the Pacific

    Euro Surveill

    (2009)
  • WW Thompson et al.

    Estimates of US influenza-associated deaths made using four different methods

    Influenza Other Respi Viruses

    (2009)
  • RE Serfling

    Methods for current statistical analysis of excess pneumonia-influenza deaths

    Public Health Rep

    (1963)
  • L Simonsen et al.

    A method for timely assessment of influenza-associated mortality in the United States

    Epidemiology

    (1997)
  • SS Shrestha et al.

    Estimating the burden of 2009 pandemic influenza A (H1N1) in the United States (April 2009–April 2010)

    Clin Infect Dis

    (2011)
  • C Viboud et al.

    Preliminary estimates of mortality and years of life lost associated with the 2009 A/H1N1 pandemic in the US and comparison with past influenza seasons

    PLoS Curr

    (2010)
  • CC Wielders et al.

    The burden of 2009 pandemic influenza A(H1N1) in the Netherlands

    Eur J Public Health

    (2012)
  • V Charu et al.

    Mortality burden of the A/H1N1 pandemic in Mexico: a comparison of deaths and years of life lost to seasonal influenza

    Clin Infect Dis

    (2011)
  • Disease and injury country estimates

  • NMR Unit-6

    Burden of illness and risk factors for transmission of seasonal influenza in four distinct regions of Peru

  • D Bandaranayake et al.

    Risk factors and immunity in a nationally representative population following the 2009 influenza A(H1N1) pandemic

    PLoS One

    (2010)
  • WA Brooks et al.

    Influenza is a major contributor to childhood pneumonia in a tropical developing country

    Pediatr Infect Dis J

    (2010)
  • G Kuan et al.

    The Nicaraguan pediatric dengue cohort study: study design, methods, use of information technology, and extension to other infectious diseases

    Am J Epidemiol

    (2009)
  • AM Presanis et al.

    The severity of pandemic H1N1 influenza in the United States, from April to July 2009: a Bayesian analysis

    PLoS Med

    (2009)
  • AM Presanis et al.

    Changes in severity of 2009 pandemic A/H1N1 influenza in England: a Bayesian evidence synthesis

    BMJ

    (2011)
  • Cited by (0)

    View full text